Since this is a response, the top bit of the opcode SHOULD be set, making the opcode value 128. However, the document shows "OP = 0", which is a mistake. In all responses the top bit of the opcode SHOULD be set. However, some implementations have the same mistake as the document and do not do this. Others sensibly set the top bit since this is a reply. Hence, clients sending PCP requests to NAT-PMP servers should expect to receive both forms of reply.